A conventional wiper control device is known which injects LED light or the like to a detection surface provided on a windshield, receives its reflected light by a light receiving element and controls a wiper based on an output signal of the light receiving element. In this device, the rainfall state is estimated based on the output signal of the light receiving element, and the wiping state (continuous wiping, intermittent wiping, etc.) of the wiper is determined. For example, in JP-A-4-349053, the rainfall state is divided into 7 steps, and an intermittence time is set to each of the steps (FIG. 1).
Also, as another prior art, a method for detecting dynamic adhesion of raindrops (JP-A-2001-180447) and a method for evaluating fluctuation of an output signal of a light receiving element (JP-A-2002-277386) are presented by the inventors. Also, as conventional examples of a method for detecting raindrops, a method for detecting the raindrop in comparison with a reference value (so-called threshold value method) (JP-A-61-37560, for example) and a method for detecting raindrops by an integrated value of the light receiving element output (so-called integration method) (JP-A-4-349053, for example) are disclosed.
In the meantime, since the detection surface is provided on the windshield in these devices, an installation position of the detection surface (that is, a device) is limited by designability and visibility of the driver. That is, the device should be installed at a position different from the driver's vision. Also, the device is more preferable if the size is smaller, but it makes the design difficult. In addition to the above problems, the size of the detection surface is reduced. That is because expansion of the area for detection requires a high performance (high brightness) light emitting source or a large light-emitting energy, which increases a unit price of components.
Thus limited installation position is often located at the position where raindrops are difficult to adhere. Moreover, in proportion to the reduced detection area, adhesion probability of raindrops on the detection surface is lowered. In addition to the lowered adhesion probability, hitting of raindrops on the detection surface is changed. In concrete, the raindrops hitting is influenced by uneven raindrop density, vehicle running speed, wind direction, etc.
Because of the above reasons, even if the same rainfall situation for a driver continues, the raindrop adhesion situation on the detection surface might be changed. For example, when the raindrop density is uneven, there is a case where a large amount of raindrops adhere at one timing while no raindrop adhere at the next timing, which can cause frequent switch-over of wiping state of the wiper in a short time. Such a state that, despite the constant rainfall situation for a driver, switch-over of wiping state (particularly, waiting time of the wiper) frequents due to limitation of adhesion probability and the wiper operation is not stable is called hunting.
On the driver side, if it is determined that constant rainfall situation has been continuing, wiping at the same stable rhythm is desirable. Therefore, such switch-over of wiping state of the wiper does not fit the sense of the driver but it is bothersome.
In the meantime, in the wiper control, capability to follow rapid increase of rainfall or temporary deterioration of visibility should be ensured. For example, when a large amount of water drops adhere on the windshield temporarily due to temporary increase of rainfall or splashing of the opposite vehicle, required wiping should be performed quickly to secure the visibility. Therefore, if the switch-over of the wiping state is simply delayed, necessary wiping operation can not be obtained in some cases.